1. Field of the Invention
This invention relates generally to a method and apparatus for delivering electrical energy produced by a defibrillator to a patient experiencing ventricular fibrillation ("VF"), and more particularly to a method and apparatus for controlling the delivery of electrical energy produced by an external defibrillator. The circuit of this invention allows for active and passive protection of the high energy delivery circuit in the event of a fault condition. The circuit also enables the patient to be protected from high voltage when the device is in standby or monitoring mode. The circuit provides a reliable and safe means of protecting the H-bridge from an over-current condition while increasing patient and operator safety. The circuit also has the advantage of being simple and inexpensive while maintaining a high degree of effectiveness.
2. Description of the Prior Art
Each day thousands of Americans are victims of cardiac emergencies. Cardiac emergencies typically strike without warning, oftentimes striking people with no history of heart disease. The most common cardiac emergency is sudden cardiac arrest ("SCA"). It is estimated that more than 1000 people per day are victims of SCA in the United States alone; this translates into one death every two minutes.
SCA occurs when the heart stops pumping blood. Usually SCA is due to abnormal electrical activity in the heart, resulting in an abnormal rhythm (arrhythmia). One such abnormal rhythm, VF, is caused by abnormal and very chaotic electrical activity in the heart. During VF the heart cannot pump blood effectively. VF may be treated by applying an electric shock to the patient's heart through the use of a defibrillator. The shock clears the heart of the abnormal electrical activity (in a process called "defibrillation") by depolarizing a critical mass of myocardial cells to allow spontaneous organized depolarization to resume, thus restoring normal function. Because blood may no longer be pumping effectively during VF, the chances of surviving decrease with time after the onset of the emergency. Brain damage can occur after the brain is deprived of oxygen for four to six minutes.
External defibrillators send electrical pulses to the patient's heart through electrodes applied to the patient's torso. External defibrillators are typically located and used in hospital emergency rooms, operating rooms, and emergency medical vehicles. Of the wide variety of external defibrillators currently available, automatic and semi-automatic external defibrillators (AEDs) are becoming increasingly popular because they can be used by relatively inexperienced personnel. Such defibrillators can also be especially lightweight, compact, and portable.
AEDs must include circuitry capable of handling the high voltages and high currents associated with electrical defibrillation. In some instances, suitable components with the required electrical characteristics are not readily available, and the AED designer must instead rely on multiple component configurations where, functionally, a single component would suffice.
Additionally, AEDs require monitoring and control circuitry to protect the patient, as well as the AED circuitry itself, in the event of a fault condition. One common fault condition occurs as a result of variations in load impedances, such as those resulting from short circuits or open circuit conditions. The high voltages applied to patients may also create situations, such as arcing between electrodes or arcing between patient wires, that could also lead to failure of the therapy electronics if not properly protected. Such monitor and control circuitry is made increasingly complex by the multiple component configurations included in currently available AEDs.
One method employed by currently available defibrillators to solve this problem is by measuring patient impedance using a low-level signal prior to delivering a shock. The disadvantage of this method is that it relies heavily on the accuracy of the low-level signal measurement relative to the actual impedance (i.e., impedance detected during the high voltage pulse delivered during defibrillation). As will be appreciated by those of skill in the art, the low-level signal cannot predict all behaviors of the external circuit during defibrillation. An example of a condition that cannot be predicted is arcing.
Another method, employed by the ForeRunner.RTM. (manufactured by Heartstream, Inc., Seattle, Wash.), is to measure impedance during the initial portion of the waveform and to allow the circuit to continue if impedance is within tolerable limits. Toward that end a 20.OMEGA. resistor is placed in series for the first 100 .mu.s that the voltage is delivered. During that time, the resistance across the electrodes is tested to ensure that the connection has not been shorted by monitoring the voltage across a 0.05.OMEGA. current sense resistor. Providing a resistance in series during the initial voltage delivery, ensures that the circuit will not be subjected to excessive current in the event that there is a short condition. However, if a fault occurs after the first 100 the circuit could be exposed to excessive currents.
What is needed, therefore, is an AED with a fault protection circuit that is capable of actively protecting the high voltage H-bridge. Protection of the H bridge can be accomplished by switching the bridge off during a fault condition, and/or passively protecting the high voltage bridge, e.g. by allowing the circuit to tolerate the fault condition. Further what is needed is a way to protect the H-bridge from valid load conditions while minimizing the exposure of the patient, or patient simulated load, to the energy stored in the AED. Finally what is needed is a way to protect the operator and/or patient in the event of a discharge to an abnormally high patient load.